Acceleration sensor with redundant contact holes

ABSTRACT

An acceleration sensor includes a mass and a supporting member linked by a flexible beam. A strain detector having low-resistance areas at both ends is formed near a boundary between the beam and the mass or between the beam and the supporting member A dielectric film formed on the supporting member and the beam has multiple contact holes disposed over each low-resistance area. Wiring formed on the dielectric film is connected to the low-resistance areas through the contact holes. The provision of multiple contact holes for each low-resistance area extends the life of the acceleration sensor by preventing sensor failure due to the separation or other failure of any single contact.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional application of application Ser. No. 11/325,336,filed on Jan. 5, 2006, which is hereby incorporated by reference in itsentirety for all purposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a three-axis acceleration sensor, moreparticularly to technology for preventing faulty electrical contacts atthe acceleration detection sites.

2. Description of the Related Art

Technology for detecting acceleration on three mutually orthogonal axesis described in, for example, Japanese Patent Application PublicationNo. 2004-198243. A piezoelectric acceleration sensor of the typedisclosed in this Publication is shown schematically in FIGS. 1 and 2.FIG. 1 shows a plan view; FIG. 2 shows a sectional view through lineI1-I2 in FIG. 1.

This acceleration sensor has a thin square first silicon layer 10, athick square second silicon layer 20, and a bonding layer 30 by whichthe first and second silicon layers are joined. Substantially L-shapedopenings 11 at the inside four corners of the first silicon layer 10create a frame-shaped peripheral attachment section 12, a square massattachment section 13 located centrally inside the peripheral attachmentsection 12, and four thin elongate beams 14 that link the massattachment section to the peripheral attachment section. The four beams14 are oriented in the x-axis and y-axis directions, which correspond tothe lateral and longitudinal directions in the plane of the drawingsheet in FIG. 1. A pair of piezoelectric resistive elements 15-1, 15-2are formed on the surface of each beam 14. The whole surface of thebeams 14, including the piezoelectric resistive elements 15-1, 15-2, iscovered with an interlayer dielectric film (not shown), on which metalwiring (also not shown) is formed. The metal wiring is electricallyconnected to the piezoelectric resistive elements 15-1, 15-2 throughcontact holes formed in the interlayer dielectric film.

The second silicon layer 20 comprises a peripheral frame 21 formed belowthe peripheral attachment section 12 in the first silicon substrate 10,surrounding a cavity 22 that extends in the vertical or z-axis directionclear through the second silicon layer 20 below the openings 11 andbeams 14. A solid rectilinear mass 23 is formed below the massattachment section 13, surrounded by the cavity 22. The height of theframe 21 in the z-axis direction exceeds the height of the mass 23. Theupper surface of the frame 21 is bonded through the bonding layer 30 tothe bottom surface of the peripheral attachment section 12, and theupper surface of the mass 23 is bonded through the bonding layer 30 tothe bottom surface of the mass attachment section 13. The bottom surfaceof the frame 21 is bonded to a base 31.

The four beams 14 allow the mass 23 to sway or move in the x-axis,y-axis, and z-axis directions. When the sensor is accelerated, the mass23 is displaced by a force proportional to the acceleration, the beams14 bend, and the resulting strain changes the electrical resistance ofthe piezoelectric resistive elements 15-1, 15-2. The change is detectedfrom signals routed through the piezoelectric resistive elements via thecontact holes and the wiring. Suitable signal processing yieldsmeasurements of acceleration on each of the three axes.

When the beams 14 bend, the greatest stress occurs at the boundaries P1between the peripheral attachment section 12 and the beams and theboundaries P2 between the mass attachment section 13 and the beams.Therefore, to maximize the sensitivity of the sensor, the piezoelectricresistive elements 15-1, 15-2 must be disposed near the boundaries P1,P2, where they will experience the greatest strain. At these locations,however, a large mechanical stress also acts on the interface betweenthe wiring in the contact holes and the silicon surfaces of thepiezoelectric resistive elements 15-1, 15-2, and can produce suchunwanted effects as wire peeling and separation, which degrade thereliability and shorten the life of the sensor.

Given the trend toward smaller sensor form factors, the contact holes infuture products can be expected to become smaller, so that the contactresistance will increase. Failure modes occurring within the contactholes (such as, for example, voids in metal wiring and silicon nodulesor aggregates) will also become more likely to affect the contactresistance, and variations in contact resistance will becomeincreasingly prevalent. Since the output current produced by apiezoelectric resistive element is very small, the contact resistancetolerance value is also small. Variations in contact resistance cantherefore quickly push the resistance value above the tolerance level,so that the sensor does not perform as designed.

These problems have a serious effect on sensor life and reliability. Inaddition, the anticipated reduction of contact hole sizes and theattendant increased variation in contact resistance values will have aserious effect on future sensor performance.

SUMMARY OF THE INVENTION

A general object of the present invention is to provide an accelerationsensor that is both sensitive and reliable.

A more specific object is to enhance the sensor's immunity to electricalfaults in contact holes formed in high-stress areas.

The invented acceleration sensor includes a mass, a supporting membersurrounding the mass, and a beam that flexibly links the mass to thesupporting member. A strain detector is formed as a single continuousunit with one end positioned on the beam and the other end positioned onthe mass or the supporting member. The strain detector is covered by adielectric film. The dielectric film has a plurality of contact holesdisposed over the end of the strain detector disposed on the beam.Wiring is formed on the dielectric film and in the contact holes, makingelectrical contact with the strain detector.

The invented acceleration sensor has an extended life expectancy becauseit can survive an electrical fault in one contact hole withoutsignificant alteration of its electrical characteristics, providedadequate electrical contact is maintained in at least one other contacthole. The provision of multiple contact holes also facilitates themeeting of contact resistance tolerances.

The contact holes are preferably aligned in the longitudinal directionof the beam, so that stress from the bending of the beam will cause afailure in only one contact hole at a time. This arrangement alsoenables the width of the beams to be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

In the attached drawings:

FIG. 1 is a plan view and FIG. 2 is a sectional view of a conventionalpiezoelectric three-axis acceleration sensor;

FIG. 3 is a plan view of a piezoelectric three-axis acceleration sensorembodying the invention;

FIG. 4 is a sectional view of the piezoelectric three-axis accelerationsensor through line I11-I12 in FIG. 3;

FIG. 5 is an enlarged sectional view showing the piezoelectric resistiveelement at the upper right in FIG. 4;

FIG. 6 is a plan view of the piezoelectric resistive element in FIG. 5;

FIG. 7 is a plan view and FIGS. 8, 9, and 10 are sectional views of thepiezoelectric resistive element in FIG. 5, illustrating the positionalrelationships among the peripheral attachment section, beam, andpiezoelectric resistive element, and illustrating tensile andcompressive strain;

FIGS. 11 and 12 are a plan view and a sectional view of a conventionalpiezoelectric resistive element;

FIGS. 13, 14, and 15 are plan views showing alternative positions of thepiezoelectric resistive element in FIG. 7;

FIG. 16 is a sectional view of the piezoelectric resistive element inFIG. 5, illustrating wiring separation in one contact hole;

FIG. 17 is a sectional view of an alternative piezoelectric resistiveelement, illustrating wiring separation in one contact hole; and

FIG. 18 is a sectional schematic view of another alternativepiezoelectric resistive element, illustrating wiring separation in onecontact hole.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the invention will now be described with reference tothe attached drawings, in which like elements are indicated by likereference characters.

Referring to FIG. 4, a piezoelectric three-axis acceleration sensor isformed by etching a silicon-on-insulator wafer comprising a firstsilicon layer 40 approximately five to ten micrometers (5 to 10 □m)thick and a second silicon layer 50 approximately 525 □m thick joined bya bonding layer 60 such as a buried silicon oxide layer (BOX layer)approximately 5 □m thick. The first silicon layer 40 is a semiconductorsilicon layer doped with an n-type impurity and has a volume resistivityof about six to eight ohm-centimeters (6 to 8 Ω·cm). The second siliconlayer 50 is a silicon substrate layer having a volume resistivity ofabout 16 Ω·cm.

Referring to FIG. 3, the first silicon layer 40 for one accelerationsensor is substantially square with sides approximately two and a halfmillimeters (2.5 mm) long, comprising L-shaped openings 41 near the fourinner corners, a frame-like peripheral attachment section 42, asubstantially square mass attachment section 43 in the center, and fourelongate beams 44 approximately 400 □m in width, oriented in the x-axisand y-axis directions of the plane of the drawing sheet, linking theperipheral attachment section 42 and the mass attachment section 43.Pairs of piezoelectric resistive elements 45-1, 45-2 that vary inelectrical resistance when strained are formed in the first siliconlayer 40, at the upper surface of the first silicon layer 40 in FIG. 5,near the boundaries P11 between the beams 44 and the peripheralattachment section 42 and the boundaries P12 between the beams 44 andthe mass attachment section 43. In each beam 44, piezoelectric resistiveelement 45-1 is disposed near (preferably straddling) boundary P11 andpiezoelectric resistive element 45-2 is disposed near (preferablystraddling) boundary P12.

The second silicon layer 50 comprises a peripheral frame 51 formed belowthe peripheral attachment section 42 in the first silicon layer 40,surrounding a cavity 52 that extends in the vertical or z-axis directionclear through the second silicon layer 50 below the openings 41 andbeams 44. A solid rectilinear mass 53 is formed below the massattachment section 43, surrounded by the cavity 52. The upper surface ofthe frame 51 is bonded through the bonding layer 60 to the bottomsurface of the peripheral attachment section 42; the upper surface ofthe mass 53 is bonded through the bonding layer 60 to the bottom surfaceof the mass attachment section 43. The height of the frame 51 in thez-axis direction exceeds the height of the mass 53 by approximately 5□m. The frame 51 and peripheral attachment section 42 constitute asupporting member that supports the beams 44. The bottom surface of theframe 51 is bonded by an adhesive or the like to a base 61 such as thefloor of a package in which the sensor is enclosed.

Referring to FIG. 5, the first silicon layer 40 is an n-typesemiconductor layer in which the piezoelectric resistive elements 45-1,45-2 are, for example, p-type diffusion layers doped with ions of boronor another p-type impurity, formed at positions astride the boundariesP11, P12 respectively (FIG. 5 shows only one piezoelectric resistiveelement 45-1). Each piezoelectric resistive element 45-1 or 45-2 has apair of low-resistance areas 46-1, 46-2 doped at high concentration withions of boron or another p-type impurity, for example, formed at its twoends. An interlayer dielectric film 47 such as a silicon oxide film isformed on the whole surface of the first silicon layer 40 including thepiezoelectric resistive elements 45-1, 45-2 and their low-resistanceareas 46-1, 46-2. The interlayer dielectric film 47 has a plurality ofcontact holes 47 a (two, for example, as shown in FIG. 6) disposed aboveeach low-resistance area 46-1 or 46-2. Each contact hole 47 a is asquare with sides approximately 1 to 5 □m long. This size enables thecontact resistance in a single contact hole to be low enough to meet theresistance tolerance of the sensor. A film of aluminum or another metalis deposited on the interlayer dielectric film 47 in the areas above thebeams 44 and the peripheral attachment section 42, and is patterned toform wiring 48 that is electrically connected to the low-resistanceareas 46-1, 46-2 via the contact holes 47 a. The whole surface of theinterlayer dielectric film 47, including the wiring 48, is covered witha protective film 49 such as a silicon nitride film.

The acceleration sensor described above is fabricated by, for example,the following process.

First, a silicon-on-insulator (SOI) wafer is obtained. The waferincludes the first silicon layer 40, the second silicon layer 50, andthe bonding layer 60.

A first silicon oxide film is formed on the surface of the first siliconlayer 40 by high-temperature thermal oxidation in a wet atmosphere.Openings are formed at predetermined positions in the first siliconoxide film by photolithography and etching, and ions of a p-typeimpurity such as boron are implanted through the openings to form p-typediffusion layers in the first silicon layer 40, which become thepiezoelectric resistive elements 45-1, 45-2. Subsequently, a secondsilicon oxide film is formed on the whole surface of the first siliconlayer 40 by thermal oxidation. Openings are then formed at predeterminedpositions in the second silicon oxide film by photolithography andetching, and ions of a p-type impurity such as boron are implanted witha high concentration through the openings to form low-resistance p-typediffusion layers, which become the low-resistance areas 46-1, 46-2.

An interlayer dielectric film 47 such as a silicon oxide film or otherfilm is formed on the whole surface by chemical vapor deposition (CVD).Two contact holes 47 a are formed in the interlayer dielectric film 47above each of the low-resistance areas 46-1, 46-2 by photolithographyand etching. A metal layer such as an aluminum layer is sputtered ontothe entire surface of the interlayer dielectric film 47, filling thecontact holes 47 a. The metal layer is then patterned byphotolithography and etching to form the wiring 48, which is therebyelectrically connected to the piezoelectric resistive elements 45-1,45-2 via the contact holes 47 a and the low-resistance areas 46-1, 46-2.After that, a protective film 49 such as a silicon nitride film or otherfilm is formed on the whole surface by plasma reactive deposition (PRD).

A layer of photoresist is applied to the surface of the protective film49, and then the openings 41 are formed by photolithography and etchingto define the peripheral attachment section 42, the mass attachmentsection 43, and the beams 44.

A third silicon oxide film is now formed by CVD on the reverse side ofthe SOI wafer, that is, the surface of the second silicon layer 50. Thecentral portion of the third silicon oxide film is removed byphotolithography and etching, while the periphery of the third siliconoxide film is left intact beneath the frame 51. The second silicon layer50 is sculpted in two etching steps to form the cavity 52 and the mass53, using the third silicon oxide film as an etching mask in theperiphery during both etching steps and another film as an etching maskin the center during the first etching step.

After that, the acceleration sensor chip is diced from the SOI wafer,and is bonded by an adhesive or the like onto a base 61 such as theinner floor of a package. Finally, wire bonding and other necessarysteps are performed to complete the fabrication process.

In this acceleration sensor, the four beams 44 can flex and twist,enabling the mass 53 to move as a whole in the z-axis direction and tosway in the x-axis and y-axis directions. These motions cause variouscombinations of tensile and compressive strain in the beams 44, whichare detected as variations in the resistance values of the piezoelectricresistive elements 45-1, 45-2. Suitable processing of signals obtainedfrom the piezoelectric resistive elements 45-1, 45-2 via the contactholes 47 a and the wiring 48 enable acceleration to be measured on threemutually orthogonal.

FIGS. 7, 8, 9, and 10 illustrate the strain detector 45-1 at a boundaryP11 between the peripheral attachment section 42 and a beam 44. When themass 53 in FIG. 4 is displaced by the force of acceleration, the beams44 bend most at this boundary P11, and at the boundary P12 where thebeams 44 are attached to the mass 53. FIGS. 9 and 10 illustrate bendingat boundary P11. Upward displacement of the beam 44 compresses thesurface of the piezoelectric resistive element 45-1 as shown in FIG. 9,whereas downward displacement of the beam 44 stretches the surface ofthe piezoelectric resistive element 45-1 as in FIG. 10. The maximumtension or compression is produced if the piezoelectric resistiveelement 45-1 is formed at the location shown, crossing the boundary P11but with most of its length disposed within the beam 44, so that one endof the piezoelectric resistive element 45-1 is disposed in theperipheral attachment section 42 near the boundary P11. This location ofthe piezoelectric resistive element 45-1 results in maximum total strainin response to acceleration, and thus a maximum change in the electricalresistance by which the acceleration is measured. The maximum sensorsensitivity is accordingly obtained.

Similarly, the optimal position for the piezoelectric resistive element45-2 at the other end of the beam 44 is astride the boundary P12 betweenthe mass 53 and beam, with most of the length of the piezoelectricresistive element 45-2 lying within the beam, so that one end of thepiezoelectric resistive element 45-2 is disposed in the mass attachmentsection 43 near the boundary P12.

Although placing the piezoelectric resistive elements 45-1, 45-2 astridethese boundaries with one end near the boundary produces maximum sensorsensitivity, it also places the electrical contact at that end at thegreatest risk of strain-induced failure, including such failure modes aswire peeling and separation. The risk is particularly high if, as shownin FIGS. 11 and 12, each of the low-resistance areas 46-1, 46-2 at theends of the piezoelectric resistive element 45-1 is connected throughonly one contact hole 47 a to the wiring 48, which is the case in theprior art.

The risk of electrical contact failure can be reduced by positioning thepiezoelectric resistive elements 45-1, 45-2 as shown in FIG. 13 or, to alesser extent, as in FIG. 14 or 15.

The piezoelectric resistive element 45-1 in FIG. 13 is centered on theboundary P11 so that both of its ends are comparatively far from theboundary. A significant portion of the length of the piezoelectricresistive element 45-1 is then disposed in the peripheral attachmentsection 42, however, well removed from the boundary P11, and experiencescomparatively little tension and compression under acceleration.

The piezoelectric resistive element 45-1 in FIG. 14 is disposed entirelywithin the beam 44, but this position completely misses the site ofmaximum tension and compression, which is at the boundary P11. Moreover,one end of the piezoelectric resistive element 45-1 is still near theboundary P11, and is therefore still at risk of strain-inducedelectrical contact failure.

The piezoelectric resistive element 45-1 in FIG. 15 is disposed entirelywithin the peripheral attachment section 42, so that for most of itslength it experiences comparatively little tension and compression underacceleration. Moreover, the end of the piezoelectric resistive element45-1 near the boundary P11 is still at approximately the same risk ofstrain-induced failure as in FIG. 7.

Accordingly, when sensitive acceleration sensing is required, the bestlocation for a piezoelectric resistive element 45-1 is the locationshown in FIGS. 5 and 7, with one end of the piezoelectric resistiveelement 45-1 astride the boundary P11.

The low-resistance area 46-2 at this end is not designed to straddle theboundary P11, but it is so close that, given the imperfect accuracy ofcurrent photolithography and etching technology, it may end up astridethe boundary P11 because of a slight mask alignment error or etchingerror. These same sources of error also mandate the proximity of thedesign position of the low-resistance area 46-2 to the boundary P11,because if the design position of the low-resistance area 46-2 were tobe shifted a little more to the right in FIG. 5, for example, thenfabrication inaccuracy might move it farther to the right, resulting inan undesirable positioning of the type shown in FIG. 13.

If fabrication inaccuracy moves the low-resistance area 46-2 slightly tothe left in FIG. 5, the situation illustrated in FIG. 16 may occur, inwhich the low-resistance area 46-2 is right on the boundary P11. Becausethere are two contact holes 47 a above this low-resistance area 46-2,however, and the two contact holes 47 a are aligned in the longitudinaldirection of the beam 44, only one of the two contact holes 47 a is onthe boundary P11. Stress at the boundary P11 may produce peeling in thiscontact hole 47 a, but the interface between the wiring 48 and thelow-resistance area 46-2 remains intact in the other contact hole, whichis disposed slightly to the right of the boundary P11. Moreover, becauseeach contact hole 47 a provides sufficient contact area between thewiring 48 and the low-resistance area 46-2 to meet the contactresistance tolerance, the sensor performance does not change. That is,the failure of the electrical contact in one contact hole 47 a does notshorten the life of the sensor.

FIG. 17 illustrates an alternative design in which the two contact holes47 a above the low-resistance area 46-2 in FIG. 16 are merged into asingle elongated contact hole 47 a. This design is inferior to thedesign in FIG. 16 for the following reason.

In FIGS. 16 and 17, once peeling starts at a contact hole, repeatedepisodes of acceleration cause the peeling to progress. This occurs inthe structures in both FIGS. 16 and 17, but there is a difference in therate of progress. The interface between the low-resistance area 46-2 andwiring 48 in FIG. 17 is a single flat plane, on which peeling canprogress easily and comparatively rapidly. In FIG. 16, the interface isinterrupted by the presence of the interlayer dielectric film 47 betweenthe two contact holes 47 a, and the undersurface of the wiring 48 doesnot have a single flat profile. The peeling process stops when itencounters the interlayer dielectric film 47 between the two contactholes 47 a, and does not readily resume in the second contact hole. InFIG. 17, even if the acceleration sensor has passed its initial factorytests without problem, the wiring 48 may peel completely away from thelow-resistance area 46-2 fairly quickly, as shown, abruptly ending theuseful life of the sensor. This is less likely to happen in FIG. 16.

A similar situation obtains in the piezoelectric resistive element 45-2disposed on the boundary P12 between the beam 44 and the mass 53.

By placing the piezoelectric resistive elements 45-1 and 45-2 atpositions astride the boundaries P11 and P12 and providing multiplecontact holes 47 a (two in the drawings, but three or more are possible)aligned longitudinally at each end of the piezoelectric resistiveelements 45-1, 45-2, the present embodiment provides the followingeffects.

(a) Extended Sensor Life

The factors that limit the life of an acceleration sensor are breakingof the flexible beams 44 and wiring degradation. A particularlyimportant factor is degradation of wiring interfaces at electricalcontacts near the highly stressed sites where the beams flex the most.The present embodiment extends sensor life by providing a plurality ofcontact holes 47 a (two, or three or more) for each electrical contactarea, preferably disposed in single file in the longitudinal directionof each beam 44.

The reason why this extends the sensor life is that even it goodelectrical contact is lost in one contact hole 47 a, the remainingcontact holes 47 a can provide sufficient electrical contact to enablethe sensor to keep operating with unaltered electrical characteristics.To obtain this effect, the low-resistance areas 46-1, 46-2 must beheavily doped, so that their electrical resistance is much lower thanthe resistance of the other parts of the piezoelectric resistiveelements, and so that they offer low contact resistance to the wiring 48in the contact holes 47 a. Each low-resistance area 46-1, 46-2 must alsobe continuous; that is, the multiple contact holes 47 a at each end ofthe piezoelectric resistive element must share the same low-resistancearea 46-1 or 46-2 at that end.

If the low-resistance areas 46-1, 46-2 are not continuous, that is, ifeach contact hole 47 a terminates on a separate low-resistance diffusionlayer 46-1 a, 46-1 b, 46-2 a, 46-2 b as shown in FIG. 18, then when aconduction failure occurs due to, for example, wire peeling at theinterface between the wiring 48 and low-resistance area 46-1 b, theintended effective length L1 of the piezoelectric resistive element 45-1changes to a longer length L2, altering the electrical characteristicsof the sensor so that it does not perform as designed.

In other words, to enable the sensor to survive the loss of goodelectrical contact in any one contact hole, it is necessary to designthe sensor so that such a loss of contact will have no significantimpact on the sensor's response to acceleration.

Aligning the contact holes 47 a at each end of the piezoelectricresistive element parallel to the width instead of the length of thepiezoelectric resistive element can also be expected to extend thesensor life. The boundaries P11, P12, where the maximum bending stressoccurs, are also parallel to the width of the piezoelectric resistiveelement, however, so parallel alignment in this direction would allowall of the contact holes 47 a to be damaged at the same time. To avoidthis situation, alignment in the longitudinal direction is preferred.

(b) Reduction of Sensor Size

When future semiconductor acceleration sensors are considered, as withother semiconductor devices, a reduction in size is a natural trend.Aligning the plurality of the contact holes 47 a longitudinally is alsoan effective strategy in this regard.

A reduction in a semiconductor acceleration sensor size means areduction in the size of the mass 53, so to obtain performanceequivalent to the performance before the reduction, it is necessary forthe beams 44 to be made more flexible. Typically, this means that thebeams 44 must have a narrower width. That may not be practical, however,if the beams 44 must be wide enough to accommodate a singlecomparatively large contact hole at each end of each piezoelectricresistive element. Wiring related issues (the wire formed on a contacthole 47 a must generally be wider than the contact hole 47 a) poseparticular problems in this respect. An effective method of obtainingthe desired beam width is to reduce the size of each contact hole 47 a,but to increase the number of the contact holes 47 a so as to obtain thesame aggregate contact resistance as before the reduction, and align thecontact holes 47 a in the longitudinal direction of the beams.

(c) Enhanced Immunity to Contact Hole Problems

Contact hole problems that afflict other semiconductor devices,including problems such as patterning failures (failure to formopenings), formation of silicon nodules, solid phase epitaxial growth,and so on, can also occur in acceleration sensors, and can alterelectrical contact resistance, resulting in inadequate sensorperformance. Increasing the number of the contact holes 47 a from one totwo or more reduces the effect of the occurrence of these problems,because the sensor can tolerate the loss or degradation of contact inany one contact hole.

Variations

The present invention is not limited to the above embodiment; variousmodifications are possible, including the following.

(1) The essential elements in FIGS. 3 to 7 are the mass (including themass attachment section), the supporting member surrounding the mass, abeam flexibly linking the mass to the supporting member, a continuouslow-resistance area disposed near a boundary between the beam and themass or a boundary between the beam and the supporting member, adielectric film having a plurality of contact holes disposed over thecontinuous low-resistance area, and wiring formed on the dielectricfilm, connected to the continuous low-resistance area through thecontact holes. Provided these elements remain present, the configurationshown in FIGS. 3 to 7 may be modified in many ways.

(2) The contact holes may have different sizes.

(3) Contact holes formed near the mass may have a different size fromcontact holes formed near the supporting member.

(4) While the simple provision of a plurality of contact holes at eachend of each piezoelectric resistive element is in itself effective, agreater effect can be obtained if the contact holes are aligned in thelongitudinal direction of the beam.

(5) In each beam, just one of the piezoelectric resistive elements 45-1,45-2 may be disposed on the boundary P11 or P12, and its position may beadjusted to adjust the relative sensitivity in the x-axis, y-axis, andz-axis directions.

(6) The acceleration sensor may comprise a semiconductor material otherthan silicon, and its shape is not limited to a square. For example, theacceleration sensor may be rectangular or circular. The shape of themass, the number of beams supporting the mass, and the number ofpiezoelectric resistive elements per beam may also be modified.

Those skilled in the art will recognize that further modifications arepossible within the scope of the invention, which is defined in theappended claims.

1. An acceleration sensor comprising: a mass; a supporting membersurrounding the mass; a beam flexibly linking the mass to the supportingmember; a strain detector formed as a single continuous unit on thesupporting member and the beam, the strain detector having a first endformed on the beam and a second end formed on the supporting member; adielectric film at least covering the strain detector, the dielectricfilm having a plurality of first contact holes disposed over the firstend of the strain detector and a second contact hole disposed over thesecond end of the strain detector; first wiring formed on the beam andin the first contact holes, making electrical contact with the straindetector; and second wiring formed on the supporting member and in thesecond contact hole, making electrical contact with the strain detector.2. The acceleration sensor of claim 1, wherein the dielectric film has aplurality of second contact holes disposed over the second end of thestrain detector, said second contact hole being one of said plurality ofsecond contact holes, and the second wiring is formed in the pluralityof second contact holes, making electrical contact therethrough with thestrain detector.
 3. The acceleration sensor of claim 1, wherein thefirst contact holes have different sizes.
 4. The acceleration sensor ofclaim 1, wherein the strain detector includes a low-resistance area andthe first wiring makes electrical contact with the low-resistance areaof the strain detector.
 5. The acceleration sensor of claim 1, whereinthe first contact holes are mutually aligned in a longitudinal directionof the beam.
 6. An acceleration sensor comprising: a mass; a supportingmember surrounding the mass; a beam flexibly linking the mass to thesupporting member; a strain detector formed as a single continuous uniton the mass and the beam, the strain detector having a first end formedon the beam and a second end formed on the mass; a dielectric film atleast covering the strain detector, the dielectric film having aplurality of first contact holes disposed over the first end of thestrain detector and a second contact hole disposed over the second endof the strain detector; first wiring formed on the beam and in the firstcontact holes, making electrical contact with the strain detector; andsecond wiring formed on the mass and in the second contact hole, makingelectrically contact with the strain detector.
 7. The accelerationsensor of claim 6, wherein the dielectric film has a plurality of secondcontact holes disposed over the second end of the strain detector, saidsecond contact hole being one of the plurality of second contact holes,and the second wiring is formed in the plurality of second contactholes, making electrical contact therethrough with the strain detector.8. The acceleration sensor of claim 6, wherein the first contact holeshave different sizes.
 9. The acceleration sensor of claim 6, wherein thestrain detector includes a low-resistance area and the first wiringmakes electrical contact with the low-resistance area of the straindetector.
 10. The acceleration sensor of claim 6, wherein the firstcontact holes are mutually aligned in a longitudinal direction of thebeam.
 11. An acceleration sensor comprising: a mass; a supporting membersurrounding the mass; a beam flexibly linking the mass to the supportingmember, the beam being joined to the supporting member at a firstboundary and to the mass at a second boundary; a first strain detectorformed as a single continuous unit on the supporting member and thebeam, the first strain detector having a first end formed on the beamand a second end formed on the supporting member; a second straindetector formed as a single continuous unit on the mass and the beam,the second strain detector having a first end formed on the beam and asecond end formed on the mass; a dielectric film at least covering thefirst strain detector and the second strain detector, the dielectricfilm having a plurality of first contact holes disposed over the firstend of the first strain detector and a plurality of second contact holesdisposed over the first end of the second strain detector; and firstwiring formed on the beam and in the first contact holes and the secondcontact holes, making electrical contact with the first strain detectorand the second strain detector.
 12. The acceleration sensor of claim 11,wherein the dielectric film has a third contact hole disposed over thesecond end of the first strain detector and a fourth contact holedisposed over the second end of the second strain detector, theacceleration sensor further comprising: second wiring formed on thesupporting member and in the third contact hole, making electricalcontact with the first strain detector; and third wiring formed on themass and in the fourth contact hole, making electrical contact with thesecond strain detector.
 13. The acceleration sensor of claim 11, whereinthe dielectric film has a plurality of third contact holes disposed overthe second end of the first strain detector and a plurality of fourthcontact hole disposed over the second end of the second strain detector,the acceleration sensor further comprising: second wiring formed on thesupporting member and in the third contact holes, making electricalcontact with the first strain detector; and third wiring formed on themass and in the fourth contact holes, making electrical contact with thesecond strain detector.
 14. The acceleration sensor of claim 11, whereinthe first contact holes have different sizes and the second contactholes have different sizes.
 15. The acceleration sensor of claim 11,wherein the first strain detector includes a first low-resistance areaand the first wiring makes electrical contact with the firstlow-resistance area of the first strain detector.
 16. The accelerationsensor of claim 11, wherein the second strain detector includes a secondlow-resistance area and the first wiring makes electrical contact withthe second low-resistance area of the second strain detector.
 17. Theacceleration sensor of claim 11 wherein the first contact holes aremutually aligned in a longitudinal direction of the beam and the secondcontact holes are mutually aligned in the longitudinal direction of thebeam.